1. Field of the Invention
The present invention relates to an optical modulator, a bias control circuit therefor, and an optical transmitter including the optical modulator.
2. Description of the Related Art
In an optical fiber communication system, a modulation rate is increasing with an increase in capacity of the system. In direct intensity modulation of a laser diode, wavelength chirping is a problem. The chirping causes waveform distortion when an optical signal passes an optical fiber having chromatic dispersion. From a standpoint of fiber loss, the most desirable wavelength to be applied to a silica fiber is 1.55 .mu.m. At this wavelength, a normal fiber has a chromatic dispersion of about 18 ps/km/nm, which limits a transmission distance. To avoid this problem, an external modulator has increasingly been expected.
As a practical external modulator, a Mach-Zehnder type optical modulator (LN modulator) using LiNbO.sub.3 (lithium niobate) as a substrate has been developed. Continuous-wave light (CW light) having a constant intensity from a light source is supplied to the LN modulator, in which a switching operation using interference of light is carried out to obtain an intensity-modulated optical signal.
The LN modulator has a frequently pointed-out defect that it causes operation point drift. To cope with the operation point drift, light output from the LN modulator is monitored, and control for operation point stabilization is carried out according to an electrical signal obtained as the result of this monitoring.
FIG. 1 is a plan view of a conventional modulator chip in an LN modulator. This modulator chip has an optical waveguide structure 4 provided by a dielectric chip 2. The dielectric chip 2 is formed of lithium niobate. In this case, the optical waveguide structure 4 is obtained by thermal diffusion of Ti (titanium).
The optical waveguide structure 4 has an input port 6 for receiving an input beam from a light source (not shown) and an output port 8 for outputting a modulated optical signal. The optical waveguide structure 4 further has a first Y branch 10 and a second Y branch 12 respectively connected to the input port 6 and the output port 8, and first and second paths 14 and 16 for connecting the Y branches 10 and 12.
The input beam supplied to the input port 6 is branched into first and second beams substantially equal in optical power to each other by the first Y branch 10. The first and second beams are guided by the paths 14 and 16, respectively, and then interfere with each other at the second Y branch 12. According to a phase difference between the first and second beams at the second Y branch 12, switching is carried out between a coupling mode where an output beam is obtained at the output port 8 and a leaky mode where a leaky beam is radiated from the second Y branch 12 into the dielectric chip 2, thereby outputting an intensity-modulated optical signal from the output port 8.
To change the phase difference between the first and second beams, a grounding electrode 18 is provided on the first path 14, and a signal electrode 20 is provided on the second path 16. The signal electrode 20 is configured as a traveling wave type such that an input end 20A is connected to an internal conductor of a connector 22 and an output end 20B is connected to an internal conductor of a connector 24. Shields of the connectors 22 and 24 and the grounding electrode 18 are grounded. The electrodes 18 and 20 are formed by vapor deposition of Au (gold), for example. Although not shown, a single or plural stabilizing buffer layers formed of Si and/or SiO.sub.2 may be provided between the dielectric chip 2 and the electrodes 18 and 20.
Operation point drift will now be described with reference to FIG. 2. In an LN modulator, an operation characteristic curve is drifted by a temperature change or aged deterioration in general (which is referred to as operation point drift). In FIG. 2, reference numerals 26 and 28 denote an operation characteristic curve and an output optical signal waveform, respectively, in the case that no operation point drift occurs, and reference numerals 30 and 32 denote an operation characteristic curve and an output optical signal waveform, respectively, in the case that an operation point drift toward positive voltage occurs. Reference numeral 34 denotes a waveform of an input signal or modulating signal (drive voltage).
The operation characteristic curve is represented as a periodic change in output optical power with an increase in voltage. In the example shown, the periodic change is given by a sine curve. Accordingly, by using voltages V0 and V1 respectively providing a minimum value and a maximum value of the optical power, respectively corresponding to the two logical values (the high level and low level) of the input signal as a binary signal to thereby perform effective switching between the coupling mode and the leaky mode mentioned above, efficient binary modulation can be performed.
When the voltages V0 and V1 are constant upon occurrence of the operation point drift, the extinction ratio of the output optical signal is degraded as shown by reference numeral 32 by the periodicity of the operation characteristic curve. Accordingly, when the operation point drift occurs in an amount of dV, the voltages V0 and V1 must be changed to (V0+dV) and (V1+dV), respectively, thereby compensating for the operation point drift.
FIG. 3 is a block diagram of a conventional optical transmitter (optical modulator) designed so as to effect operation point stabilization. CW light as an input beam from a laser diode (LD) 36 is supplied to the input port 6 of the modulator chip 2 shown in FIG. 2, for example. An output beam from the output port 8 of the modulator chip 2 is divided into two branch beams by an optical coupler 38. One of the two branch beams is launched into an optical fiber transmission line (not shown), and the other branch beam is supplied to a photodetector (PD) 40. The photodetector 40 is provided by a photodiode, for example. In this case, the photodetector 40 outputs a current signal. Therefore, this current signal from the photodetector 40 is converted into a voltage signal by a current/voltage (I/V) converter 42. Thereafter, the voltage signal output from the I/V converter 42 is supplied through a bandpass filter 44 to a phase comparator circuit 46.
A low-frequency signal (pilot signal) output from an oscillator 48 is used for operation point stabilization. The pilot signal is supplied to the phase comparator circuit 46 and a drive circuit 50. The drive circuit 50 may be composed of a variable-gain amplifier for amplifying a data input signal and a low-pass filter connected to the output of the variable-gain amplifier. In this case, the gain of the variable-gain amplifier is changed by the low-frequency signal, and as a result, the low-frequency signal is superimposed on the data input signal. By the use of the low-pass filter, the low-frequency signal is superimposed on both the low level and high level of the data input signal in opposite phases. A resultant signal is then supplied as a modulating signal to the connector 22 of the modulator chip 2.
The phase comparator circuit 46 is provided by a synchronous detector circuit, for example. The phase comparator circuit 46 performs phase comparison between the low-frequency signal from the oscillator 48 and a low-frequency component from the photodetector 40. The result of this phase comparison appears in a DC component of an output signal from the phase comparator circuit 46. Then, the bias circuit 52 performs feedback control of a bias voltage to be supplied to the connector 24 of the modulator chip 2, according to the DC component. In this feedback loop, the bias voltage is adjusted so that the low-frequency component from the photodetector 40 is minimized.
Referring to FIG. 4, there is shown the principle of the operation point stabilization in the optical modulator shown in FIG. 3. Reference numeral 54 denotes a waveform of the input electrical signal, that is, a waveform of the modulating signal output from the drive circuit 50. An optimum operation point is determined by an operation characteristic curve such that both levels of the input electrical signal 54 provide maximum and minimum output optical powers as shown by reference numeral 56. When the characteristic curve is shifted as shown by reference numeral 58 or 60 along the voltage axis because of variations in temperature or the like, a low-frequency component is generated in the output optical signal, and the direction of shifting is reflected by the phase of the low-frequency component. That is, the phases of envelopes of the output optical signals provided by the characteristic curves 58 and 60 are different 180.degree. from each other. Accordingly, the operation point is stabilized by performing synchronous detection with the phase comparator circuit 46 as shown in FIG. 3.
According to the conventional bias control technique for operation point stabilization as described above with reference to FIGS. 3 and 4, the low-frequency signal must be superimposed on the data input signal. Accordingly, there is a possibility of intersymbol interference on a main signal. Furthermore, the two levels of the input electrical signal must be made correspond to the minimum value and the maximum value of the optical output power. Accordingly, the amplitude of the input electrical signal is so limited as to correspond to a voltage (V.pi. voltage) given by the difference between a voltage giving the maximum optical power and a voltage giving the minimum optical power in the operation characteristic curve. That is, in the case that the amplitude of the input electrical signal is smaller than the V.pi. voltage, the extinction ratio of the output optical signal is degraded. Because a small value of the amplitude of the input electrical signal is suitable for achievement of a high modulation rate, the problem of degradation in the extinction ratio is serious in providing an optical modulator suitable for high-speed operation.